This invention relates generally to a method for driving a liquid crystal matrix display and more particularly to a method for driving a liquid crystal matrix display which reduces the problem of crosstalk without substantially increasing energy requirements.
Recently, the replacement of conventional cathode ray tubes with liquid crystal matrix displays for use in small size television sets has received much attention. The use of liquid crystal technology as a display device in such items as electronic timepieces is well known. However, when a liquid crystal matrix is used as a display device in a television, the liquid crystals are required to operate at much higher performance levels and under more exacting conditions. In order to obtain the high resolution required for displaying an acceptable television image while using liquid crystal technology, a large number of picture elements or pixels are required. Each pixel must exhibit high contrast characteristics, quick response time and a large angle of visibility. In addition, the liquid crystals used in liquid crystal televisions must have stable temperature and frequency characteristics.
Several methods for driving such liquid crystal matrix display devices have been considered including: a generalized AC amplitude selective multiplexing method, a two-frequency drive method, and a switch-matrix drive method. At the present time however, only the generalized AC amplitude selective multiplexing method has been put into practical use since the other methods consume larger amounts of energy and have higher manufacturing costs.
One problem that occurs when using liquid crystal technology in television displays is crosstalk. In a liquid crystal device, by charging the opposed electrodes of the liquid crystal, a field is set up within the crystal which aligns the crystalline structure to permit the liquid crystal device to be transmissive or opaque. However, due to the nature of the crystalline structure, when a liquid crystal device has been activated to a state of opacity, a finite amount of time is required for the crystal to revert to transparency once the charge on the electrodes has been removed. When used in devices such as timepieces or calculators which require low resolution and which are not display critical, this crystal display lag time is not disruptive to viewing. However, when used as an active display device such as in a television receiver, the high scanning rates associated with each liquid crystal pixel, when coupled with crystal display lag, creates the crosstalk problem. In particular, if a liquid crystal matrix is arranged in an XY grid and a series of continuous low frequency row scanning signals are provided for each pixel sequentially, each pixel being selected by applying a voltage to the pixel's column line at a suitable time, then the crosstalk phenomenon will occur to varying degrees in each pixel which is adjacent to or substantially near a pixel which is intended to be activated and which has previously been activated. This is because the scanning rate associated with the liquid crystal matrix display is faster than the time required for each liquid crystal pixel to return to a neutral state.
In the prior art a method is known for inverting the signal applied to each liquid crystal pixel during the time it is activated in order to reduce the problem of crosstalk. This is known as the high frequency driving method. However, the high frequency driving method requires large energy consumption due to the amount of signal switching that is required and thus creates a power problem when used in portable battery operated liquid crystal matrix display televisions.
Accordingly it is desirable to provide an improved method for driving a liquid crystal matrix display especially for use in a television which reduces the above noted problem of crosstalk without substantially increasing energy requirements.